Milk is a strategic beverage in the field of Italian and world-wide beverage equipment. While milk in Italy plays a significant role limited generally to breakfasts (cappuccino, espresso macchiato and the like), it is used abroad for making many other beverages that are served also after the main meals.
Milk is often “foamed” to make a final two-phase product consisting of a creamy part and of a liquid part which are not immediately separable from each other. A typical example in which frothing the milk is required is the one of preparing the cappuccino. The cappuccino is indeed defined as a hot beverage consisting of a dose of espresso coffee (weighing about 23 g) to which a dose of foamed hot milk is added. This mixture forms the final beverage weighing about 110-120 g.
Normally, the “frothing” operation of the milk is carried out by hand, directly by the barista, through the use of a steam nozzle. This operation requires some attention. Indeed, the serum-proteins that are in the milk are surface-active but thermolabile compounds. By decreasing the surface tension of the fluid and therefore the surface rigidity thereof, the surface activity promotes the frothing of the milk if air is introduced. The thermolability instead causes the existence of a limit temperature, which in the specific case is about 70° C., at which the proteins deteriorate and lose the surface-active behaviour thereof. This deterioration process is an irreversible process: by cooling the milk, the proteins do not reacquire their functional surface-active feature.
The presence of the aforesaid limit temperature is well known to the experts operators in the field. Vice versa, when operators with little experience encounter difficulties in obtaining the frothing of the milk, they normally insist on using the steam nozzle in an attempt to obtain the frothing desired. By doing this, the milk heats past the limit temperature and often is brought to boiling. The increase of the temperature and the breakdown of a sizeable portion of the protein material on the one hand result in an increase of the surface tension and on the other, the boiling of the milk, which results in the formation of a froth consisting of very large bubbles which tend to disappear very quickly.
To avoid these situations, which are not improbable in the technical field of the preparation of hot beverages, various solutions have been introduced which tend to automate the frothing process, with or without the aid of an operator. For example, documents EP 1 501 398 B1 and EP 1 776 905 B1 describe respective improved steam nozzles in which both air, coming from a compressor, and steam, coming from a boiler, are introduced. Although it resolves the manual frothing problem, thus avoiding the overheating of the milk, this solution may not be applied in those specific situations in which the frothing is required of a well-defined dose of milk. A typical case is the one of apparatuses for dispensing beverages operating in self-service mode, typically common in the hotel field.
Various solutions have been developed to deal with the aforesaid specific situations. For example, document EP 0 243 326 B1 describes a device for frothing the milk through the use of the Venturi effect. Although it resolves the problem of the preset dose of milk to be foamed, this device, under particular operating conditions, sometimes has instabilities caused by the reaching of temperatures which may induce the boiling of the milk and the expansion of the drawn air, thus causing a slowing down of the flow with subsequent reduction of the dose in the cup.
Good stability was obtained with the application of a pump, as described in document EP 1 785 074 B1. The milk is forcedly pushed due to the pump, thus forcing the apparatus to always dispense the same dose.
In apparatuses with dosed frothing like the ones described above, the frothing of the milk occurs due to the effect of the air which, in certain cases, is mixed with the milk simultaneously with the steam, and in other cases, is introduced into the flow of milk already heated by the steam coming from the boiler. For the frothing quality of the milk, it may then be suitable to froth a quantity of “cold” milk, then heat the product obtained up to its optimal temperature, which for the Italian cappuccino is about 65° C.
A cold frothing and successive heating process has been known for a long time and is described for example, in document EP 0 485 350 A1. More recently, this process was illustrated also in documents EP 1 593 330 B1 and EP 2 120 656 B1. In this context, a specific quantity of milk taken from a refrigeration unit, is drawn and mixed by a pump with a certain quantity of air. The pump is preferably a gear pump and provides to finely mix the two fluids, so as to obtain a high density milk cream and to send the foamed milk into the cup. In the case of a cold beverage, the sending into the cup occurs directly, or through a further cooling element. In the case of a hot beverage, the sending into the cup occurs through a heating device.
The foamed milk is heated by means of a heat exchanger for the preparation of a hot beverage in the examples mentioned. The use of the heat exchanger however has certain drawbacks. The heating occurs by heat exchange between the walls of the heat exchanger and the flow of milk. This means that in order to quickly transfer the heat to the foamed milk, there is a need to heat the walls of the heat exchanger to a much higher temperature than the final temperature of the milk. This fact leads to the formation on the heat exchanger walls of solid casein deposits (the so-called “milkstone”) which alter the taste of the milk and which, in order to be removed, require energetic and frequent washings.
In the case of an apparatus in which there is provided also the dispensing of cold milk, the preparation of a cold beverage immediately following the preparation of a hot beverage may not be carried out with the correct temperature, since the heat exchangers of the type described in documents EP 0 485 350 A1 and EP 1 593 330 B1 are necessarily provided with a significant mass. Indeed, the heat exchanger which has not yet cooled down releases heat to the milk that it comes in contact with. Therefore, there is a need to wait for the cooling of the heat exchanger, or to force a quick cooling thereof, or again to deviate the flow of milk thus avoiding the heat exchanger, but significantly complicating the circuit and thus compromising the features of hygienic quality and washability thereof.
By taking advantage of the increased latent heat of condensation, the steam heating instead allows heating large quantities of milk in a few moments with an increased energy efficiency, and also preserving the flavour qualities of the milk.
Document DE 20 2014 010272 U1 describes an apparatus for heating and frothing milk in which at least one part of the components adapted for the operation of the apparatus itself—such as e.g. the milk pump and the separation valve—is not positioned in a cooled environment, such as e.g. a refrigerator. Also in this case therefore, the preparation of a cold beverage immediately following the preparation of a hot beverage is not always carried out with the correct temperature. Further apparatuses for heating and frothing milk are described for example, in documents U.S. Pat. No. 6,099,878 A and EP 2 060 211 A1.
Finally, document WO 2013/076634 A1 describes an automatic beverage dispenser and a respective system for the production of hot water. The automatic beverage dispenser is provided with a shape memorizing thermostatic valve designed to mitigate the temperature of the hot water without the use of electric power. The thermostatic valve identifies the temperature of the hot and cold water and thermostatically controls the outlet water. No electronic control is provided.